I haven't stayed here for a while, but the elite treatment will be quite different - the Courtyard has a lounge with evening snacks and nibbles, the Mira traditionally didn't even offer elites breakfast (though there have been reports that it now does for a "trial period", as it is a design hotel it isn't obligated to do so). The locations are also fairly different, between HK Island and Kowloon. Personally I would prefer the Courtyard over the Mira as you are more likely to get a better view (of water), and the lounge with evening snacks and drinks can be fun - you can get likely get breakfast at both (but with the Courtyard it will be in the lounge). As a design hotel the Mira is less inclined to offer you benefits than the Courtyard. The location in Sheung Wan is also better for a tourist and most business visits, unless you intend to spend your time primarily in TST/Kowloon shopping.

Either way, enjoy your stay in HK.